1. Field of the Invention
The present invention relates to gas chromatography systems for generally continuously sampling chemical samples and detecing desired compounds therefrom. In particular, the subject invention is directed to gas chromatographic column assemblies for temperature programmed analyses. More in particular, the subject invention relates to gas chromatograph column assemblies where power savings are achieved through optimized packing of capillary gas chromatograph column members with temperature sensors and heating wires which substantially increases the internal contact of such components with themselves and each other while reducing the amount of surface area of these components in contact with the surroundings. Still further, this invention relates to gas chromatograph systems which include assemblies of capillary gas chromatograph column members, temperature sensing mechanisms and heating mechanisms formed into a gas chromatograph column assembly which is positionally located in a manner to optimize thermal effects and produce an overall low power consumption system.
2. Prior Art
High performance gas chromatography has typically required the use of large laboratory instruments using large amounts of electrical power in their operation. This is especially the case for the standard practice of temperature programming a chromatographic separation in which the temperature of the oven containing the gas chromatography column is steadily increased to extend the range of gas chromatography separation capability. The large power required to heat and temperature program gas chromatography ovens has limited the capability of gas chromatography for use in portable instrumentation and especially in hand-portable instrumentation used in the field. Without large external power sources or large batteries, gas chromatography design an operations have been limited largely to non-temperature programming applications in small, lightweight portable instruments.
Additonal requirements for gas chromatography technology to be practical in small portable instruments is for the technology to be compatible with and use commercially available gas chromatography capillary column technology. Since the use of capillary columns has become standard practice in laboratory gas chromatography instrumentation, a large number of capillary columns are now commercially available which offer a wide range of separation capabilities. The wide range of separation capabilities has been made possible through variation of the chemical compositions of the polymers which coat the inner walls of the capillary gas chromatography columns. Choice may now be had from many polymer coatings that are commercially available in capillary gas chromatography columns having standard hicknesses, column lengths, and column inner diameters to optimize the chemical separations required of the gas chromatography. The ability to readily use this commercially available gas chromatography column technology in small portable gas chromatography instruments is desirable for the practical realization of similar analytical capabilities in portable or small gas chromatography instrument.
The temperature programming of capillary gas chromatography columns is standardly practiced by electronic control of the temperature of an oven containing the gas chromatography column. To achieve rapid and uniform temperature response of the gas chromatography column assembly to temperature changes in the oven, capillary gas chromatography columns are standardly packaged by winding the columns on a wire frame support. The winding of the columns on the wire frame support provides extensive surface contact of the capillary gas chromatography column with the heated air in th oven for rapid temperature equilibration of the capillary gas chromatography column with the oven air. In laboratory gas chromatography ovens, the air within the oven is typically mixed with a fan to achieve temperature uniformity within the oven. Laboratory gas chromatography instrument typically consume power on the order of kilowatts for temperature programming and are power limited to temperature ramping rates approximating 10's of .degree.C./min or less, especially at higher operating temperatures. While smaller, more portable gas chromatography instruments have been manufactured which have smaller ovens, such still require powers on the order of 1 kW or more for temperature programming, especially when ramping rates of 10's of .degree.C./min are required for fast analysis times.
Reductions in gas chromatography oven size to that of a small heated compartment large enough to contain a short length of gas chromatography column have been made for the purposes of reducing power consumption and reducing instrument size. The resulting gas chromatography instrument are typically operated isothermally to avoid the power consumption associated with temperature programming, but this greatly constrains the analytical capabilities of such gas chromatography instrumentation. One case in which low power temperature programming has been implemented is described by Maswadeh et al. in "New Generation of Hand-Held, Compact, isposable Gas Chromatography Devices," Field-Portable Chromatography and Spectrometry workshop, Jun. 3-5, 1996, Snowbird, UT, pp. P56-P59. In this case a palm-size gas chromatography module was demonstrated which consumes 15 W of power for temperature programming at a 0.75.degree. C. /s ramp rate. A short ramp with a maximum temperature of 60.degree. C. served to limit power consumption by the module.
The need for fast temperature programming of miniature chromatographic analysis instrumentation is described by Sides and sates in U.S. Pat. No. 5,014,541. They describe the requirement to raise the temperature of the capillary gas chromatography column from 50.degree. C. to 120.degree. C. within 20 seconds to achieve their analysis objectives. They accomplish this with a miniature gas chromatography column assembly in which the standard gas chromatography oven is replaced by a tubular heat conductor support on which the gas chromatography column is wound. A heating element within the tubular support is used for temperature programming. While this instrument achieves a gas chromatograph of small size, the power consumptica is on the order of 1 kW and a portable power generator is a recommended option for portable operation of this commercial instrument.
The importance of reducing the thermal mass of gas chromatography column assemblies for the rapid heating was recognized by Norem in U.S. Pat. No. 3,159,996. This invention consisted of a glass tube with three parallel bores and sufficient length to contain a heater wire, a resistance thermometer wire (a type of temperature sensor), with the remaining bore coated on the inside to function as a gas chromatography column. While such a device could have a smaller thermal mass than a small, conventional gas chromatography oven, a large amount of power will still be required to heat the sizable mass of glass tubing.
Another way to significantly reduce power consumption with a miniature gas chromatography is by reducing the electrical heating and sensing elements of gas chromatography ovens to miniature forms and integrate them with a capillary gas chromatography column. U.S. Pat. No. 5,005,399 achieves this by using a thin-film coated capillary gas chromatography column wound on a mandrill consisting of an insulating material. Electrical current passed through the thin film surrounding the gas chromatography column is used to resistively heat the column. While this approach uses far less power than heating a conventional gas chromatography oven, it till requires significant power to heat the gas chromatography column because of the large surface area of the gas chromatography column in contact with the mandrill material since this insulating support is also heated through contact with the heating element. A serious shortcoming of this approach is the difficulty of fabricating annular thin film coatings of substantial length having sufficient uniformity and freedom from defects. The thermal mass of the gas chromatography column plus contacting insulation is large enough that currents on the order of 1 ampere or more are typically required for fast temperature programming using battery voltages such as 12 volts. The conduction of such large currents in thin film resistive heaters requires the films to be substantially free of defects. While thickness non-uniformity results in uneven heating, typical defects in thin films due to microcontamination, particulates, abrasion from handling, or stresses and fractures due to bending (such as coiling the column) cause local hot spots and thin film breakdown resulting in failure of the heater element. A further difficulty with this approach is that it is not compatible with the use of commercially available gas chromatography column technology; special thin film resistor-coated versions of each gas chromatography column of interest would have to be manufactured requiring large scale, omnidirectonal precision deposition of very high quality films. Given the technical problems with the handling, operating and manufacturing of precision thin film resistive heaters of this scale, this is a serious difficulty.
While not claiming low power operation, a similar approach by U.S. Pat. No. 4,726,822 relies on annular thin film heater and insulator layers to miniaturize a heated capillary gas chromatography column assembly. In addition to the handling, operating and manufacturing impracticalities discussed in the preceding paragraph with respect to U.S. Pat. No. 5,005,399, the close spacing of multiple thin films present additional difficulties.
Another approach to reducing power consumption by a capillary gas chromatography column assembly has been published by Holland, et al. in "Handheld Gas Chromatography Instrumentation for Chemical Weapons Convention Treaty Verification Instrumentation," Field Screening Methods for Hazardous Xastes and Toxic Chemicals, Vol. 1, Air & Waste Management Association, Pittsburgh, 1995, pp. 229-235. In this approach a gas chromatography capillary column is placed inside a length of small plastic tubing along with coaxial heater and sensor wires. This gas chromatography assembly uses far less power than the power required by commercial gas chromatography ovens. Typical powers required for temperature programming this assembly are still on the order of 10's of watts per meter of column length for fast, short gas chromatography column configurations reported by Overton and Carney in "New Horizons in Gas Chromatography: Field Applications of Microminiaturized Gas Chromatographic Techniques," Trends in Analytical Chemistry, Vol. 13, 1994, pp. 252-25, and by Overton, et al. in "A New Portable Micro Gas Chromatograph for Environmental Analysis," in Field Screening Methods for Hazardous Wastes and Toxic Chemicals, Vol. 1, Air & Waste Management Association, Pittsburgh, 1995, pp. 207-21. Much lower power consumption is required for battery powered fast temperature programming by small portable gas chromatography instruments. While this approach permits the use of commercially available gas chromatography capillary columns, the difficulty of threading capillary gas chromatography columns, heater wires, and sensor wires into small plastic tubing limits the practical assembly lengths to several meters.